Recently, in consideration of environmental issues, vehicles using electric motors as driving power sources such as electric vehicles, hybrid vehicles and fuel cell electric vehicles have been attracting attention. Such a vehicle is provided with an electric storage unit formed of a secondary battery or a capacitor, for converting kinetic energy to electric energy at the time of regenerative braking and for storing the same.
In such a vehicle having the electric motor as the driving power source, it is desirable to increase charge/discharge capacity of the electric storage unit, in order to attain higher running performance such as acceleration performance and per-charge mileage. As a method of increasing the charge/discharge capacity of the electric storage unit, a structure including a plurality of electric storage units has been proposed.
By way of example, U.S. Pat. No. 6,608,396 discloses an electrical motor power management system for providing desired high DC voltage level to a high voltage vehicle traction system. The electrical motor power management system includes: a plurality of power stages for providing DC power to at least one inverter, each stage including a battery and boost/buck DC-DC converter and connected in parallel; and a controller controlling the plurality of power stages such that batteries to the plurality of power stages are uniformly charged/discharged so that the plurality of power stages maintain output voltage to the at least one inverter.
Now, driving force required of a vehicle significantly varies dependent on the running condition. For instance, when the vehicle runs at a low speed or runs downhill, the required electric power is smaller than the sum of tolerable discharge electric powers of the plurality of electric storage units. In such a situation, it is desired to selectively stop voltage converting operation of the voltage converting unit (corresponding to the boost/buck DC-DC converter mentioned above) corresponding to a prescribed electric storage unit, thereby to reduce power conversion loss at the voltage converting unit.
In a power system including a plurality of electric storage units and a plurality of corresponding voltage converting units, each of the voltage converting units must perform the voltage converting operation in accordance with the status value of corresponding electric storage unit, and therefore, these converting units are configured to execute voltage converting operations in mutually independent control systems. Specifically, there are a plurality of control systems corresponding to the voltage converting units. Provision of the plurality of control systems allows execution of voltage converting operation at an optimal timing by each of the voltage converting units.
It is common to provide in each control system with a control element such as an integral element, which outputs a control value determined dependent on history. Such a control element calculates a control value in accordance with status value at each time point as well as with variation in status values in the past. Therefore, a power system involving intermittently executed voltage converting operations is susceptible to disturbance derived from such a control element.
Specifically, a control value output immediately after the voltage converting operation is resumed is influenced by status values that should not be used for calculation, such as variation in status values during last voltage converting operation or variation in status values occurred while the voltage converting operation was suspended.
As a result, before and after switching of the voltage converting units for executing the voltage converting operation, output voltage from a voltage converting unit could be discontinuous, possibly making instable the voltage supplied from the voltage converting unit to a load device.